This invention relates to a system for improving the image brightness and image sharpness in laser television projection systems.
Laser projection uses a raster procedure to produce images. In the raster procedure, three primary colors red, green, and blue are emitted from one laser or from two or more different lasers. The three colors are modulated separately, brought together again, and scanned over a surface. The image is then formed by points, as in the standard television picture tube. The color of an image point is defined by the relative power of its primary color components. Normally the beam is deflected horizontally by a polygonal mirror and vertically by a galvanometric scanner.
The laser beam causes image irregularities (spots) or so-called speckles in the laser image projection due to the spatial coherence of the laser light. The effect can be observed by shining a diverged laser beam onto a wall. The eye of a viewer sees a disk with a stochastic light-dark distribution, instead of a uniformly illuminated field.
The spots are perceived when light is reflected off a projection surface and to the eye. The projection surface has areas which are uneven compared with the size of the light source wavelength. The light waves reflected off these uneven surface areas of the projection surface reach the eye with different phase positions creating interference that is perceived in the eye. From a given standpoint of the observer, this interference produces a spatial two-dimensional light-dark pattern.
A laser projection system is described in European Patent No. EP0589179A1 where laser radiation consisting of three primary colors is passed through a diffusion element in a common optical path. The diffusion element rotates in the range from 30 to 40 rpm to prevent the formation of spots due to interference phenomena and creates a clearer, truer-color image.
A projection light image display system with reduced spot formation is also described in European Patent No. EP0385706. A coherent light source is used to produce a light beam. A light modulator directs the light from the source onto the display screen reproducing an image. The screen is coupled with a transducer that creates acoustic surface waves in the screen where the image is reproduced. The acoustic waves possess an amplitude which is greater than the wavelength of the light beam. The surface waves are supposed to prevent the formation of spots when viewing uniform image contents taking advantage of the fact that the eye is not quick enough to perceive the moving interference pattern created by the surface waves.
A system is also described in U.S. Pat. No. 5,313,479, in which a rotating diffusion element is arranged in the optical path of the laser. The diffusion element moves the interference pattern so quickly that it cannot be perceived by the human eye.
The systems described above require mechanical transducers and, therefore, are relatively expensive. The diffusion elements mentioned above also induce lateral diffusion of photons in the screen which reduce image sharpness and brightness. If the reflectivity of a laser projection screen is low, more powerful lasers are needed which are more costly, require more power and cooling and have a shorter operating life.
Thus, a need exists for reducing interference in light image display systems while at the same time maintaining high image sharpness and brightness.
A projection surface reduces the formation of spots during laser projection while increasing the sharpness and brightness of the laser projection after being scattered by the screen. The projection surface includes scattering microparticles that are in constant motion relative to each other and a source of laser radiation of the laser projection. The microparticles reduce the formation of spots that normally occurs when viewing the contents of images which are uniformly illuminated by means of laser projection and which are reflected off a projection surface.
The projection surface is made in such a way that although the image irregularities (spots) or speckles still occur, they are variable in time so that the two-dimensional light-dark pattern, when seen over the reaction time of the eye, is averaged out. Thus, the viewer perceives a uniform brightness.
In one embodiment of the invention, the microparticles are formed in a suspension fluid. Due to Brownian molecular motion in the fluid, the microparticles continue to move. The fluid may be warmed to further increase motion of the microparticles. In another embodiment of the invention, a piezoelectric force is used to further increase motion by causing turbulence in the microparticles. It is also advantageous to constantly mix the particles to prevent settling. Both embodiments are relatively simple and are resistant to interference.
In place of a solid projection wall, the projection surface has mobile scattering centers. These scattering centers are implemented by suspending the microparticles in the fluid. The motion of the microparticles creates a large number of different light/dark patterns during illumination by the laser spot. The eye perceives the patterns as a uniform image for uniform image contents.
In another embodiment of the invention, the back wall of the screen is made reflective. The incident laser beam passes through a front window and through the suspension fluid that backscatters the light to remove stationary speckle. The light is also reflected back after hitting the back mirror. The viewer sees both the backscattered light from the incident beam as well as the forward scattered light from the reflected beam. The reflected light gives rise to further scatter and thus adds to image brightness.
In another embodiment of the invention, a cellular structure is located in the suspension fluid. The cellular structure laterally confines light increasing the sharpness of the laser light seen by the viewer. The cells can be different shapes such as, honeycomb shaped or square shaped. Different absorbing, scattering or reflective surfaces can be used on the sides of the cellular structure in contact with the suspension medium according to the desired brightness and sharpness of the laser projection.
The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings.